The IMC rating requires a specific amount of unapproved training. Any approved training can be verified and therefore there is nothing to prevent that "experience" being counted towards the total required.
The holder of a BCPL (defunct since Sept 2012) now has a UK National CPL (Art 82A) which includes the inbuilt IMC privileges (Schedule 7).
(b) any United Kingdom Basic Commercial Pilot’s Licence (Aeroplanes), other than a
licence referred to in sub-paragraph (a), is deemed to be a United Kingdom
Commercial Pilot’s Licence (Aeroplanes);
(c) any JAA Commercial Pilot Licence (Aeroplane) that is endorsed to the effect that
the licence does not fully comply with JAR-FCL is deemed to be a United
Kingdom Commercial Pilot’s Licence (Aeroplanes);